Flare stack with perforated flame holder

ABSTRACT

A flare stack burner includes a fuel stack housing in which a fuel nozzle and a perforated flame holder are positioned. The fuel nozzle is configured to emit a fuel stream toward the flame holder.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from U.S. ProvisionalPatent Application No. 62/117,887, entitled “FLARE STACK WITH PERFORATEDFLAME HOLDER,” filed Feb. 18, 2015 (docket number 2651-257-02); which,to the extent not inconsistent with the disclosure herein, isincorporated by reference.

BACKGROUND

Flare stacks are used to burn off vented volatile organic compounds. Forexample, in an oil refinery, a flare stack may be used to provideemergency burning of volatile compounds, or provide for a safe way torelieve high sudden pressure events of flammable materials. In an oilfield, a flare stack may be used to burn off natural gas that isproduced as a byproduct of crude oil production. In a landfill, a flarestack may be used to burn off methane released by decompositionprocesses. Because volatile compounds are considered pollutants and areoften flammable, it is generally considered preferable to burn thevolatile compounds, rather than to vent the volatile compounds directlyto the atmosphere. In flare stack applications, it can be important tocontrol the height of a flame envelope created by the burner. In someapplications, especially those known by the term of art “enclosedflares,” it may be required or desired that the flame not exceed theheight of the flare stack itself. By keeping the flame inside the flarestack, safety may be improved. Moreover, aesthetics may be improvedsufficiently to avoid complaints about a visible flame.

Enclosed flare stacks or ground flares can be used for burning offunusable waste field gas in a variety of oil and gas productionapplications, for example. Waste gases may be released duringover-pressuring of plant equipment. The waste gases may be transportedto a corresponding ground flare. Some ground flares are enclosed. By“enclosed” it is meant that a flame envelope is substantially blockedfrom view by persons outside a controlled access area.

Flame length may determine a required height, girth, or other dimensionsof the ground flare structure. A problem may arise when the flamebecomes visible (e.g., is too high). Excessively high flame length maysubstantially halt operation, and/or may result in fines or be expressedas greater capital cost, increased operating expenses, and/or otherremediation expenses.

SUMMARY

According to an embodiment, a device includes a housing including aninlet configured to be coupled to a waste gas supply as part of a flarestack, and an outlet configured to release products of combustion to theatmosphere. A perforated flame holder is positioned inside the housing,the perforated flame holder having a first face, a second face lyingopposite the first face, and a plurality of perforations extendingthrough the perforated flame holder between the first and second faces.A nozzle is configured to receive a flow of waste gas from the inlet andemit a waste gas stream toward the first face of the perforated flameholder. The perforated flame holder is configured to support combustionof the waste gas substantially within the plurality of perforations.

According to an embodiment, a method includes outputting a waste gas andsupplemental fuel sufficient to raise a heating value of the waste gasplus supplemental fuel to about 100 BTU per cubic foot or less toward aperforated flame holder; and combusting the waste gas and supplementalfuel substantially within a plurality of perforations extending throughthe perforated flame holder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a flare stack with a perforated flame holder,according to an embodiment.

FIG. 2 is a simplified perspective view of a burner system including aperforated flame holder, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flameholder of FIGS. 1 and 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner systemincluding the perforated flame holder of FIGS. 1, 2 and 3, according toan embodiment.

FIG. 5 is a diagrammatic side-sectional view of a portion of a flarestack that includes a perforated flame holder substantially as describedwith reference to FIGS. 2 and 3, according to an embodiment.

FIG. 6 is a diagrammatic side-sectional view of a portion of a flarestack, according to another embodiment.

FIG. 7 is a diagrammatic side-sectional view of a portion of a flarestack, according to an embodiment, that includes a retrofit burnerinstalled in a pre-existing flare stack.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here.

FIG. 1 is a diagram of a flare stack 100 with a perforated flame holder102, according to an embodiment. The flare stack 100 includes a stackstructure 104 configured to support a flare. A flare is a combustionreaction that burns off volatile compounds to control pressure insystems such as oil production, oil refining, and other chemicalprocessing systems. A volatile compound source 106 at leastintermittently outputs a flow of high vapor pressure flammablecompounds. Optionally, a pressure control valve 108 may provide aconstant pressure sink to the volatile compound source 106, anddetermine a constant pressure flow of volatile compounds to a volatilecompound nozzle 110. For some systems, this can cause a variable flowrate of volatile compounds to the volatile compound nozzle 110. Whenvolatile compound flow is sufficient, the volatile compound nozzleoutputs a stream of volatile compounds. A combustion air source 112,such as a damper or a blower, provides combustion air. The stream ofvolatile compounds flows and entrains combustion air to form a volatilecompound mixture 114. A perforated flame holder 102 is supported by aperforated flame holder support structure 222 at a position selected toreceive the volatile compound mixture 114. As described elsewhereherein, a combustion reaction supported at least partially by thevolatile compound mixture can be held by the perforated flame holder102.

According to an embodiment, a temperature-maintenance fuel nozzle 116 isconfigured to output a start-up flame or a temperature-maintenance fueland air mixture 118 to establish or maintain an operating temperature ofthe perforated flame holder 102 using fuel from a fuel source 120.

Because flow from the volatile compound source 106 can be intermittentor at least non-steady, the temperature-maintenance fuel nozzle 116 canbe configured to cooperate with the fuel source 120 to provide arelatively high fuel and air mixture 118 flow rate when the volatilecompound mixture 114 flow rate is low and provide a relatively low orzero fuel and air mixture 118 flow rate when the volatile compoundmixture 114 flow rate is high.

According to an embodiment, a controller 122 is operatively coupled to aflow sensor 124 configured to measure flow of volatile compounds fromthe volatile compound source 106. The controller can use digital logicto determine a corresponding flow rate appropriate for the fuel and airmixture 118, and control a fuel flow valve 126 to provide a selectedflow rate of fuel from the fuel source 120 to thetemperature-maintenance fuel nozzle 116.

The temperature-maintenance fuel nozzle 116 can include a fuel riser 128and an ignition source 129 configured to ignite a start-up flame nearthe temperature-maintenance fuel nozzle. The ignition source 129 caninclude a hot surface igniter, a spark-discharge igniter, or a pilotflame, for example. Additionally or alternatively, the ignition source129 may include a flame holder operable to hold a flame at a locationproximate to the fuel riser 128. The flame holder may be configured tobe actuated to selectively hold a flame at the location proximate to thefuel source or to allow fuel from the fuel source to travel to theperforated flame holder 102 for combustion. In such an embodiment, theignition source 129 may additionally include a separate igniter oralternatively the fuel riser 128 may be manually ignited at start-up.

According to an embodiment, when the ignition source 129 or actuatableflame holder is enabled, a start up flame is supported between thetemperature-maintenance fuel nozzle 116 and the perforated flame holder102. When the ignition source 129 or flame holder is not enabled, thetemperature-maintenance fuel nozzle 116 outputs a flow of the fuel andair mixture 118 to the perforated flame holder 102 for combustion in theperforated flame holder 102. The controller 122 can be operativelycoupled to the ignition source 129 to determine whether a start-up flameis supported or whether the fuel and air mixture 118 is delivered to theperforated flame holder 102 for combustion.

According to an embodiment, the temperature-maintenance fuel nozzle 116can be configured to add a relatively high BTU-content fuel, such aspropane or natural gas, to a relatively low BTU-content fuel from thevolatile compound nozzle 110. For continuous flow operations, the“temperature maintenance” performed by the temperature-maintenance fuelnozzle 116 may consist essentially of increasing the BTU content of thecombustible materials (fuel plus volatile compound) delivered to theperforated flame holder 102.

In one experiment, it was found that use of the perforated flame holder102 could reduce the necessary BTU content of methane fuel plus volatilecompound mixture from 300 BTUs per cubic foot to below 100 BTUs percubic foot while maintaining steady combustion, compared to burning thevolatile compound in a conventional flame. The capabilities of theperforated flame holder 102 can thus be used to advantage in many wastegas burn-off applications, whether or not in a contained flame flarestack, and can result in significant fuel cost savings.

During times when substantially no volatile compounds are output fromthe volatile compound source 106, the controller 122 can cause the flarestack 100 to operate in a “cold standby” state, where minimal or no fuelfrom the fuel source 120 is consumed, and the fuel control valve 126 ismaintained in an “off” position. Optionally, the system 100 can includea volatile compound flow valve 130 operatively coupled to the controller122. The controller can hold the volatile compound flow valve 130 in anoff state whenever the flare stack 100 is in a cold standby state.

When an imminent volatile compound flow is detected (e.g., by a pressuresensor (not shown) or the volatile compound flow sensor 124, thecontroller can convert the flare stack 100 to a “warm standby” state,wherein the fuel control valve 126 is opened sufficiently, and theignition source 129 enabled to support a start-up flame. The system 100can change from a warm standby state to a “hot standby” state when thetemperature of the space between the volatile compound nozzle 110 andthe perforated flame holder 102 is warmed by the start-up flame to asufficiently hot temperature to ensure complete combustion of thevolatile compound. In the hot standby state, the flare stack 100 canoperate as a normal flare stack with volatile compound flaring occurringin a conventional flame below the perforated flame holder. In somecases, the volatile compound is itself a fuel of sufficient heatingvalue to provide a continual flame without any additional or supportingfuel. In other cases, a supplemental fuel is required to raise the heatvalue of the (supplemental) fuel plus volatile compound. In still othercases, it is more proper to call the fuel an ignition fuel inasmuch asthe volatile compound has sufficient heating value to maintain thecombustion reaction, but for reasons of safety or convenience, it ispreferable to have a fuel of known composition and pressure available asan ignition source. Unless noted, the term fuel may function in any ofthese senses.

When the perforated flame holder 102 is warmed to a start-uptemperature, the controller can disable the ignition source 129 to liftfrom the start-up flame location and cause the fuel/air mixture 118 toimpinge on the perforated flame holder 102, wherein combustion is held.Simultaneously, with no ignition by the start-up flame, the volatilecompound/air mixture 114 travels to the perforated flame holder 102wherein the volatile compound is combusted.

When the volatile compound flow rate is sufficiently high to maintaincombustion in the perforated flame holder 102, the controller 122 canreduce the fuel flow rate or stop fuel flow using the fuel control valve126. Optionally, the controller 122 can include a proportionalcontroller configured to maintain a fuel mixture 118 flow rate that isinversely proportional to the volatile compound mixture 114 flow rate.

Optionally, the controller can be operatively coupled to control thecombustion air source 112. Optionally, the controller can be operativelycoupled to a sensor 132 configured to sense combustion, temperature, orother parameter related to performance of the flare stack 100.

FIG. 2 is a simplified diagram of a burner system 200 including aperforated flame holder 102 configured to hold a combustion reaction,according to an embodiment. As used herein, the terms perforated flameholder, perforated reaction holder, porous flame holder, porous reactionholder, duplex, and duplex tile shall be considered synonymous unlessfurther definition is provided. Experiments performed by the inventorshave shown that perforated flame holders 102 described herein cansupport very clean combustion. Specifically, in experimental use ofsystems 200 ranging from pilot scale to full scale, output of oxides ofnitrogen (NOx) was measured to range from low single digit parts permillion (ppm) down to undetectable (less than 1 ppm) concentration ofNOx at the stack. These remarkable results were measured at 3% (dry)oxygen (O₂) concentration with undetectable carbon monoxide (CO) atstack temperatures typical of industrial furnace applications(1400-1600° F.). Moreover, these results did not require anyextraordinary measures such as selective catalytic reduction (SCR),selective non-catalytic reduction (SNCR), water/steam injection,external flue gas recirculation (FGR), or other heroic extremes that maybe required for conventional burners to even approach such cleancombustion.

According to embodiments, the burner system 200 includes a fuel andoxidant source 202 disposed to output fuel and oxidant into a combustionvolume 204 to form a fuel and oxidant mixture 206. As used herein, theterms fuel and oxidant mixture and fuel stream may be usedinterchangeably and considered synonymous depending on the context,unless further definition is provided. As used herein, the termscombustion volume, combustion chamber, furnace volume, and the likeshall be considered synonymous unless further definition is provided.The perforated flame holder 102 is disposed in the combustion volume 204and positioned to receive the fuel and oxidant mixture 206.

FIG. 3 is a side sectional diagram 300 of a portion of the perforatedflame holder 102 of FIGS. 1 and 2, according to an embodiment. Referringto FIGS. 2 and 3, the perforated flame holder 102 includes a perforatedflame holder body 208 defining a plurality of perforations 210 alignedto receive the fuel and oxidant mixture 206 from the fuel and oxidantsource 202. As used herein, the terms perforation, pore, aperture,elongated aperture, and the like, in the context of the perforated flameholder 102, shall be considered synonymous unless further definition isprovided. The perforations 210 are configured to collectively hold acombustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel can include hydrogen, a hydrocarbon gas, a vaporizedhydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered orpulverized solid. The fuel can be a single species or can include amixture of gas(es), vapor(s), atomized liquid(s), and/or pulverizedsolid(s). For example, in a process heater application the fuel caninclude fuel gas or byproducts from the process that include carbonmonoxide (CO), hydrogen (H₂), and methane (CH₄). In another applicationthe fuel can include natural gas (mostly CH₄) or propane (C₃H₈). Inanother application, the fuel can include #2 fuel oil or #6 fuel oil.Dual fuel applications and flexible fuel applications are similarlycontemplated by the inventors. The oxidant can include oxygen carried byair, flue gas, and/or can include another oxidant, either pure orcarried by a carrier gas. The terms oxidant and oxidizer shall beconsidered synonymous herein.

According to an embodiment, the perforated flame holder body 208 can bebounded by an input face 212 disposed to receive the fuel and oxidantmixture 206, an output face 214 facing away from the fuel and oxidantsource 202, and a peripheral surface 216 defining a lateral extent ofthe perforated flame holder 102. The plurality of perforations 210 whichare defined by the perforated flame holder body 208 extend from theinput face 212 to the output face 214. The plurality of perforations 210can receive the fuel and oxidant mixture 206 at the input face 212. Thefuel and oxidant mixture 206 can then combust in or near the pluralityof perforations 210 and combustion products can exit the plurality ofperforations 210 at or near the output face 214.

According to an embodiment, the perforated flame holder 102 isconfigured to hold a majority of the combustion reaction 302 within theperforations 210. For example, on a steady-state basis, more than halfthe molecules of fuel output into the combustion volume 204 by the fueland oxidant source 202 may be converted to combustion products betweenthe input face 212 and the output face 214 of the perforated flameholder 102. According to an alternative interpretation, more than halfof the heat or thermal energy output by the combustion reaction 302 maybe output between the input face 212 and the output face 214 of theperforated flame holder 102. As used herein, the terms heat, heatenergy, and thermal energy shall be considered synonymous unless furtherdefinition is provided. As used above, heat energy and thermal energyrefer generally to the released chemical energy initially held byreactants during the combustion reaction 302. As used elsewhere herein,heat, heat energy and thermal energy correspond to a detectabletemperature rise undergone by real bodies characterized by heatcapacities. Under nominal operating conditions, the perforations 210 canbe configured to collectively hold at least 80% of the combustionreaction 302 between the input face 212 and the output face 214 of theperforated flame holder 102. In some experiments, the inventors produceda combustion reaction 302 that was apparently wholly contained in theperforations 210 between the input face 212 and the output face 214 ofthe perforated flame holder 102. According to an alternativeinterpretation, the perforated flame holder 102 can support combustionbetween the input face 212 and output face 214 when combustion is“time-averaged.” For example, during transients, such as before theperforated flame holder 102 is fully heated, or if too high a (cooling)load is placed on the system, the combustion may travel somewhatdownstream from the output face 214 of the perforated flame holder 102.Alternatively, if the cooling load is relatively low and/or the furnacetemperature reaches a high level, the combustion may travel somewhatupstream of the input face 212 of the perforated flame holder 102.

While a “flame” is described in a manner intended for ease ofdescription, it should be understood that in some instances, no visibleflame is present. Combustion occurs primarily within the perforations210, but the “glow” of combustion heat is dominated by a visible glow ofthe perforated flame holder 102 itself. In other instances, theinventors have noted transient “huffing” or “flashback” wherein avisible flame momentarily ignites in a region lying between the inputface 212 of the perforated flame holder 102 and the fuel nozzle 218,within the dilution region D_(D). Such transient huffing or flashback isgenerally short in duration such that, on a time-averaged basis, amajority of combustion occurs within the perforations 210 of theperforated flame holder 102, between the input face 212 and the outputface 214. In still other instances, the inventors have noted apparentcombustion occurring downstream from the output face 214 of theperforated flame holder 102, but still a majority of combustion occurredwithin the perforated flame holder 102 as evidenced by continued visibleglow from the perforated flame holder 102 that was observed.

The perforated flame holder 102 can be configured to receive heat fromthe combustion reaction 302 and output a portion of the received heat asthermal radiation 304 to heat-receiving structures (e.g., furnace wallsand/or radiant section working fluid tubes) in or adjacent to thecombustion volume 204. As used herein, terms such as radiation, thermalradiation, radiant heat, heat radiation, etc. are to be construed asbeing substantially synonymous, unless further definition is provided.Specifically, such terms refer to blackbody-type radiation ofelectromagnetic energy, primarily at infrared wavelengths, but also atvisible wavelengths owing to elevated temperature of the perforatedflame holder body 208.

Referring especially to FIG. 3, the perforated flame holder 102 outputsanother portion of the received heat to the fuel and oxidant mixture 206received at the input face 212 of the perforated flame holder 102. Theperforated flame holder body 208 may receive heat from the combustionreaction 302 at least in heat receiving regions 306 of perforation walls308. Experimental evidence has suggested to the inventors that theposition of the heat receiving regions 306, or at least the positioncorresponding to a maximum rate of receipt of heat, can vary along thelength of the perforation walls 308. In some experiments, the locationof maximum receipt of heat was apparently between ⅓ and ½ of thedistance from the input face 212 to the output face 214 (i.e., somewhatnearer to the input face 212 than to the output face 214). The inventorscontemplate that the heat receiving regions 306 may lie nearer to theoutput face 214 of the perforated flame holder 102 under otherconditions. Most probably, there is no clearly defined edge of the heatreceiving regions 306 (or for that matter, the heat output regions 310,described below). For ease of understanding, the heat receiving regions306 and the heat output regions 310 will be described as particularregions 306, 310.

The perforated flame holder body 208 can be characterized by a heatcapacity. The perforated flame holder body 208 may hold thermal energyfrom the combustion reaction 302 in an amount corresponding to the heatcapacity multiplied by temperature rise, and transfer the thermal energyfrom the heat receiving regions 306 to heat output regions 310 of theperforation walls 308. Generally, the heat output regions 310 are nearerto the input face 212 than are the heat receiving regions 306. Accordingto one interpretation, the perforated flame holder body 208 can transferheat from the heat receiving regions 306 to the heat output regions 310via thermal radiation, depicted graphically as 304. According to anotherinterpretation, the perforated flame holder body 208 can transfer heatfrom the heat receiving regions 306 to the heat output regions 310 viaheat conduction along heat conduction paths 312. The inventorscontemplate that multiple heat transfer mechanisms including conduction,radiation, and possibly convection may be operative in transferring heatfrom the heat receiving regions 306 to the heat output regions 310. Inthis way, the perforated flame holder 102 may act as a heat source tomaintain the combustion reaction 302, even under conditions where acombustion reaction 302 would not be stable when supported from aconventional flame holder.

The inventors believe that the perforated flame holder 102 causes thecombustion reaction 302 to begin within thermal boundary layers 314formed adjacent to walls 308 of the perforations 210. Insofar ascombustion is generally understood to include a large number ofindividual reactions, and since a large portion of combustion energy isreleased within the perforated flame holder 102, it is apparent that atleast a majority of the individual reactions occur within the perforatedflame holder 102. As the relatively cool fuel and oxidant mixture 206approaches the input face 212, the flow is split into portions thatrespectively travel through individual perforations 210. The hotperforated flame holder body 208 transfers heat to the fluid, notablywithin thermal boundary layers 314 that progressively thicken as moreand more heat is transferred to the incoming fuel and oxidant mixture206. After reaching a combustion temperature (e.g., the auto-ignitiontemperature of the fuel), the reactants continue to flow while achemical ignition delay time elapses, over which time the combustionreaction 302 occurs. Accordingly, the combustion reaction 302 is shownas occurring within the thermal boundary layers 314. As flow progresses,the thermal boundary layers 314 merge at a merger point 316. Ideally,the merger point 316 lies between the input face 212 and output face 214that define the ends of the perforations 210. At some position along thelength of a perforation 210, the combustion reaction 302 outputs moreheat to the perforated flame holder body 208 than it receives from theperforated flame holder body 208. The heat is received at the heatreceiving region 306, is held by the perforated flame holder body 208,and is transported to the heat output region 310 nearer to the inputface 212, where the heat is transferred into the cool reactants (and anyincluded diluent) to bring the reactants to the ignition temperature.

In an embodiment, each of the perforations 210 is characterized by alength L defined as a reaction fluid propagation path length between theinput face 212 and the output face 214 of the perforated flame holder102. As used herein, the term reaction fluid refers to matter thattravels through a perforation 210. Near the input face 212, the reactionfluid includes the fuel and oxidant mixture 206 (optionally includingnitrogen, flue gas, and/or other “non-reactive” species). Within thecombustion reaction region, the reaction fluid may include plasmaassociated with the combustion reaction 302, molecules of reactants andtheir constituent parts, any non-reactive species, reactionintermediates (including transition states), and reaction products. Nearthe output face 214, the reaction fluid may include reaction productsand byproducts, non-reactive gas, and excess oxidant.

The plurality of perforations 210 can be each characterized by atransverse dimension D between opposing perforation walls 308. Theinventors have found that stable combustion can be maintained in theperforated flame holder 102 if the length L of each perforation 210 isat least four times the transverse dimension D of the perforation. Inother embodiments, the length L can be greater than six times thetransverse dimension D. For example, experiments have been run where Lis at least eight, at least twelve, at least sixteen, and at leasttwenty-four times the transverse dimension D. Preferably, the length Lis sufficiently long for thermal boundary layers 314 to form adjacent tothe perforation walls 308 in a reaction fluid flowing through theperforations 210 to converge at merger points 316 within theperforations 210 between the input face 212 and the output face 214 ofthe perforated flame holder 102. In experiments, the inventors havefound L/D ratios between 12 and 48 to work well (i.e., produce low NOx,produce low CO, and maintain stable combustion).

The perforated flame holder body 208 can be configured to convey heatbetween adjacent perforations 210. The heat conveyed between adjacentperforations 210 can be selected to cause heat output from thecombustion reaction portion 302 in a first perforation 210 to supplyheat to stabilize a combustion reaction portion 302 in an adjacentperforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 canfurther include a fuel nozzle 218, configured to output fuel, and anoxidant source 220 configured to output a fluid including the oxidant.For example, the fuel nozzle 218 can be configured to output pure fuel.The oxidant source 220 can be configured to output combustion aircarrying oxygen, and optionally, flue gas.

The perforated flame holder 102 can be held by a perforated flame holdersupport structure 222 configured to hold the perforated flame holder 102at a dilution distance D_(D) away from the fuel nozzle 218. The fuelnozzle 218 can be configured to emit a fuel jet selected to entrain theoxidant to form the fuel and oxidant mixture 206 as the fuel jet andoxidant travel along a path to the perforated flame holder 102 throughthe dilution distance D_(D) between the fuel nozzle 218 and theperforated flame holder 102. Additionally or alternatively (particularlywhen a blower is used to deliver oxidant contained in combustion air),the oxidant or combustion air source can be configured to entrain thefuel and the fuel and oxidant travel through the dilution distanceD_(D). In some embodiments, a flue gas recirculation path 224 can beprovided. Additionally or alternatively, the fuel nozzle 218 can beconfigured to emit a fuel jet selected to entrain the oxidant and toentrain flue gas as the fuel jet travels through the dilution distanceD_(D) between the fuel nozzle 218 and the input face 212 of theperforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one ormore fuel orifices 226 having an inside diameter dimension that isreferred to as “nozzle diameter.” The perforated flame holder supportstructure 222 can support the perforated flame holder 102 to receive thefuel and oxidant mixture 206 at the distance D_(D) away from the fuelnozzle 218 greater than 20 times the nozzle diameter. In anotherembodiment, the perforated flame holder 102 is disposed to receive thefuel and oxidant mixture 206 at the distance D_(D) away from the fuelnozzle 218 between 100 times and 1100 times the nozzle diameter.Preferably, the perforated flame holder support structure 222 isconfigured to hold the perforated flame holder 102 at a distance about200 times or more of the nozzle diameter away from the fuel nozzle 218.When the fuel and oxidant mixture 206 travels about 200 times the nozzlediameter or more, the mixture is sufficiently homogenized to cause thecombustion reaction 302 to produce minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fueland oxidant source, according to an embodiment. A premix fuel andoxidant source can include a premix chamber (not shown), a fuel nozzleconfigured to output fuel into the premix chamber, and an oxidant (e.g.,combustion air) channel configured to output the oxidant into the premixchamber. A flame arrestor can be disposed between the premix fuel andoxidant source and the perforated flame holder 102 and be configured toprevent flame flashback into the premix fuel and oxidant source.

The oxidant source 220, whether configured for entrainment in thecombustion volume 204 or for premixing, can include a blower configuredto force the oxidant through the fuel and oxidant source 202.

The support structure 222 can be configured to support the perforatedflame holder 102 from a floor or wall (not shown) of the combustionvolume 204, for example. In another embodiment, the support structure222 supports the perforated flame holder 102 from the fuel and oxidantsource 202. Alternatively, the support structure 222 can suspend theperforated flame holder 102 from an overhead structure (such as a flue,in the case of an up-fired system). The support structure 222 cansupport the perforated flame holder 102 in various orientations anddirections.

The perforated flame holder 102 can include a single perforated flameholder body 208. In another embodiment, the perforated flame holder 102can include a plurality of adjacent perforated flame holder sectionsthat collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 222 can be configured tosupport the plurality of perforated flame holder sections. Theperforated flame holder support structure 222 can include a metalsuperalloy, a cementatious, and/or ceramic refractory material. In anembodiment, the plurality of adjacent perforated flame holder sectionscan be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W betweenopposite sides of the peripheral surface 216 at least twice a thicknessdimension T between the input face 212 and the output face 214. Inanother embodiment, the perforated flame holder 102 can have a widthdimension W between opposite sides of the peripheral surface 216 atleast three times, at least six times, or at least nine times thethickness dimension T between the input face 212 and the output face 214of the perforated flame holder 102.

In an embodiment, the perforated flame holder 102 can have a widthdimension W less than a width of the combustion volume 204. This canallow the flue gas circulation path 224 from above to below theperforated flame holder 102 to lie between the peripheral surface 216 ofthe perforated flame holder 102 and the combustion volume wall (notshown). Referring again to both FIGS. 2 and 3, the perforations 210 canbe of various shapes. In an embodiment, the perforations 210 can includeelongated squares, each having a transverse dimension D between opposingsides of the squares. In another embodiment, the perforations 210 caninclude elongated hexagons, each having a transverse dimension D betweenopposing sides of the hexagons. In yet another embodiment, theperforations 210 can include hollow cylinders, each having a transversedimension D corresponding to a diameter of the cylinder. In anotherembodiment, the perforations 210 can include truncated cones ortruncated pyramids (e.g., frustums), each having a transverse dimensionD radially symmetric relative to a length axis that extends from theinput face 212 to the output face 214. In some embodiments, theperforations 210 can each have a lateral dimension D equal to or greaterthan a quenching distance of the flame based on standard referenceconditions. Alternatively, the perforations 210 may have lateraldimension D less then than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 210has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably,each of the plurality of perforations 210 has a lateral dimension Dbetween 0.1 inch and 0.5 inch. For example the plurality of perforations210 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as thetotal volume of all perforations 210 in a section of the perforatedflame holder 102 divided by a total volume of the perforated flameholder 102 including body 208 and perforations 210. The perforated flameholder 102 should have a void fraction between 0.10 and 0.90. In anembodiment, the perforated flame holder 102 can have a void fractionbetween 0.30 and 0.80. In another embodiment, the perforated flameholder 102 can have a void fraction of about 0.70. Using a void fractionof about 0.70 was found to be especially effective for producing verylow NOx.

The perforated flame holder 102 can be formed from a fiber reinforcedcast refractory material and/or a refractory material such as analuminum silicate material. For example, the perforated flame holder 102can be formed to include mullite or cordierite. Additionally oralternatively, the perforated flame holder body 208 can include a metalsuperalloy such as Inconel or Hastelloy. The perforated flame holderbody 208 can define a honeycomb. Honeycomb is an industrial term of artthat need not strictly refer to a hexagonal cross section and mostusually includes cells of square cross section. Honeycombs of othercross sectional areas are also known.

The inventors have found that the perforated flame holder 102 can beformed from VERSAGRID® ceramic honeycomb, available from AppliedCeramics, Inc. of Doraville, S.C.

The perforations 210 can be parallel to one another and normal to theinput and output faces 212, 214. In another embodiment, the perforations210 can be parallel to one another and formed at an angle relative tothe input and output faces 212, 214. In another embodiment, theperforations 210 can be non-parallel to one another. In anotherembodiment, the perforations 210 can be non-parallel to one another andnon-intersecting. In another embodiment, the perforations 210 can beintersecting. The body 308 can be one piece or can be formed from aplurality of sections.

In another embodiment, which is not necessarily preferred, theperforated flame holder 102 may be formed from reticulated ceramicmaterial. The term “reticulated” refers to a netlike structure.Reticulated ceramic material is often made by dissolving a slurry into asponge of specified porosity, allowing the slurry to harden, and burningaway the sponge and curing the ceramic.

In another embodiment, which is not necessarily preferred, theperforated flame holder 102 may be formed from a ceramic material thathas been punched, bored or cast to create channels.

In another embodiment, the perforated flame holder 102 can include aplurality of tubes or pipes bundled together. The plurality ofperforations 210 can include hollow cylinders and can optionally alsoinclude interstitial spaces between the bundled tubes. In an embodiment,the plurality of tubes can include ceramic tubes. Refractory cement canbe included between the tubes and configured to adhere the tubestogether. In another embodiment, the plurality of tubes can includemetal (e.g., superalloy) tubes. The plurality of tubes can be heldtogether by a metal tension member circumferential to the plurality oftubes and arranged to hold the plurality of tubes together. The metaltension member can include stainless steel, a superalloy metal wire,and/or a superalloy metal band.

The perforated flame holder body 208 can alternatively include stackedperforated sheets of material, each sheet having openings that connectwith openings of subjacent and superjacent sheets. The perforated sheetscan include perforated metal sheets, ceramic sheets and/or expandedsheets. In another embodiment, the perforated flame holder body 208 caninclude discontinuous packing bodies such that the perforations 210 areformed in the interstitial spaces between the discontinuous packingbodies. In one example, the discontinuous packing bodies includestructured packing shapes. In another example, the discontinuous packingbodies include random packing shapes. For example, the discontinuouspacking bodies can include ceramic Raschig ring, ceramic Berl saddles,ceramic Intalox saddles, and/or metal rings or other shapes (e.g. SuperRaschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systemsincluding the perforated flame holder 102 provide such clean combustion.

According to an embodiment, the perforated flame holder 102 may act as aheat source to maintain a combustion reaction even under conditionswhere a combustion reaction would not be stable when supported by aconventional flame holder. This capability can be leveraged to supportcombustion using a leaner fuel-to-oxidant mixture than is typicallyfeasible. Thus, according to an embodiment, at the point where the fuelstream 206 contacts the input face 212 of the perforated flame holder102, an average fuel-to-oxidant ratio of the fuel stream 206 is below a(conventional) lower combustion limit of the fuel component of the fuelstream 206—lower combustion limit defines the lowest concentration offuel at which a fuel and oxidant mixture 206 will burn when exposed to amomentary ignition source under normal atmospheric pressure and anambient temperature of 25° C. (77° F.).

The perforated flame holder 102 and systems including the perforatedflame holder 102 described herein were found to provide substantiallycomplete combustion of CO (single digit ppm down to undetectable,depending on experimental conditions), while supporting low NOx.According to one interpretation, such a performance can be achieved dueto a sufficient mixing used to lower peak flame temperatures (amongother strategies). Flame temperatures tend to peak under slightly richconditions, which can be evident in any diffusion flame that isinsufficiently mixed. By sufficiently mixing, a homogenous and slightlylean mixture can be achieved prior to combustion. This combination canresult in reduced flame temperatures, and thus reduced NOx formation. Inone embodiment, “slightly lean” may refer to 3% O₂, i.e. an equivalenceratio of ˜0.87. Use of even leaner mixtures is possible, but may resultin elevated levels of O₂. Moreover, the inventors believe perforationwalls 308 may act as a heat sink for the combustion fluid. This effectmay alternatively or additionally reduce combustion temperatures andlower NOx.

According to another interpretation, production of NOx can be reduced ifthe combustion reaction 302 occurs over a very short duration of time.Rapid combustion causes the reactants (including oxygen and entrainednitrogen) to be exposed to NOx-formation temperature for a time tooshort for NOx formation kinetics to cause significant production of NOx.The time required for the reactants to pass through the perforated flameholder 102 is very short compared to a conventional flame. The low NOxproduction associated with perforated flame holder combustion may thusbe related to the short duration of time required for the reactants (andentrained nitrogen) to pass through the perforated flame holder 102.

FIG. 4 is a flow chart showing a method 400 for operating a burnersystem including the perforated flame holder shown and described herein.To operate a burner system including a perforated flame holder, theperforated flame holder is first heated to a temperature sufficient tomaintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 400 begins with step402, wherein the perforated flame holder is preheated to a start-uptemperature, T_(S). After the perforated flame holder is raised to thestart-up temperature, the method proceeds to step 404, wherein the fueland oxidant are provided to the perforated flame holder and combustionis held by the perforated flame holder.

According to a more detailed description, step 402 begins with step 406,wherein start-up energy is provided at the perforated flame holder.Simultaneously or following providing start-up energy, a decision step408 determines whether the temperature T of the perforated flame holderis at or above the start-up temperature, T_(S). As long as thetemperature of the perforated flame holder is below its start-uptemperature, the method loops between steps 406 and 408 within thepreheat step 402. In step 408, if the temperature T of at least apredetermined portion of the perforated flame holder is greater than orequal to the start-up temperature, the method 400 proceeds to overallstep 404, wherein fuel and oxidant is supplied to and combustion is heldby the perforated flame holder.

Step 404 may be broken down into several discrete steps, at least someof which may occur simultaneously.

Proceeding from step 408, a fuel and oxidant mixture is provided to theperforated flame holder, as shown in step 410. The fuel and oxidant maybe provided by a fuel and oxidant source that includes a separate fuelnozzle and oxidant (e.g., combustion air) source, for example. In thisapproach, the fuel and oxidant are output in one or more directionsselected to cause the fuel and oxidant mixture to be received by theinput face of the perforated flame holder. The fuel may entrain thecombustion air (or alternatively, the combustion air may dilute thefuel) to provide a fuel and oxidant mixture at the input face of theperforated flame holder at a fuel dilution selected for a stablecombustion reaction that can be held within the perforations of theperforated flame holder.

Proceeding to step 412, the combustion reaction is held by theperforated flame holder.

In step 414, heat may be output from the perforated flame holder. Theheat output from the perforated flame holder may be used to power anindustrial process, heat a working fluid, generate electricity, orprovide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Varioussensing approaches have been used and are contemplated by the inventors.

Generally, combustion held by the perforated flame holder is very stableand no unusual sensing requirement is placed on the system. Combustionsensing may be performed using an infrared sensor, a video sensor, anultraviolet sensor, a charged species sensor, thermocouple, thermopile,flame rod, and/or other combustion sensing apparatuses. In an additionalor alternative variant of step 416, a pilot flame or other ignitionsource may be provided to cause ignition of the fuel and oxidant mixturein the event combustion is lost at the perforated flame holder.

Proceeding to decision step 418, if combustion is sensed not to bestable, the method 400 may exit to step 424, wherein an error procedureis executed.

For example, the error procedure may include turning off fuel flow,re-executing the preheating step 402, outputting an alarm signal,igniting a stand-by combustion system, or other steps. If, in step 418,combustion in the perforated flame holder is determined to be stable,the method 400 proceeds to decision step 420, wherein it is determinedif combustion parameters should be changed. If no combustion parametersare to be changed, the method loops (within step 404) back to step 410,and the combustion process continues. If a change in combustionparameters is indicated, the method 400 proceeds to step 422, whereinthe combustion parameter change is executed. After changing thecombustion parameter(s), the method loops (within step 404) back to step410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if achange in heat demand is encountered. For example, if less heat isrequired (e.g., due to decreased electricity demand, decreased motivepower requirement, or lower industrial process throughput), the fuel andoxidant flow rate may be decreased in step 422. Conversely, if heatdemand is increased, then fuel and oxidant flow may be increased.Additionally or alternatively, if the combustion system is in a start-upmode, then fuel and oxidant flow may be gradually increased to theperforated flame holder over one or more iterations of the loop withinstep 404.

Referring again to FIG. 2, the burner system 200 includes a heater 228operatively coupled to the perforated flame holder 102. As described inconjunction with FIGS. 3 and 4, the perforated flame holder 102 operatesby outputting heat to the incoming fuel and oxidant mixture 206. Aftercombustion is established, this heat is provided by the combustionreaction 302; but before combustion is established, the heat is providedby the heater 228.

Various heating apparatuses have been used and are contemplated by theinventors. In some embodiments, the heater 228 can include a flameholder configured to support a flame disposed to heat the perforatedflame holder 102. The fuel and oxidant source 202 can include a fuelnozzle 218 configured to emit a fuel stream 206 and an oxidant source220 configured to output oxidant (e.g., combustion air) adjacent to thefuel stream 206. The fuel nozzle 218 and oxidant source 220 can beconfigured to output the fuel stream 206 to be progressively diluted bythe oxidant (e.g., combustion air). The perforated flame holder 102 canbe disposed to receive a diluted fuel and oxidant mixture 206 thatsupports a combustion reaction 302 that is stabilized by the perforatedflame holder 102 when the perforated flame holder 102 is at an operatingtemperature. A start-up flame holder, in contrast, can be configured tosupport a start-up flame at a location corresponding to a relativelyunmixed fuel and oxidant mixture that is stable without stabilizationprovided by the heated perforated flame holder 102.

The burner system 200 can further include a controller 230 operativelycoupled to the heater 228 and to a data interface 232. For example, thecontroller 230 can be configured to control a start-up flame holderactuator configured to cause the start-up flame holder to hold thestart-up flame when the perforated flame holder 102 needs to bepre-heated and to not hold the start-up flame when the perforated flameholder 102 is at an operating temperature (e.g., when T≧T_(S)).

Various approaches for actuating a start-up flame are contemplated. Inone embodiment, the start-up flame holder includes amechanically-actuated bluff body configured to be actuated to interceptthe fuel and oxidant mixture 206 to cause heat-recycling and/orstabilizing vortices and thereby hold a start-up flame; or to beactuated to not intercept the fuel and oxidant mixture 206 to cause thefuel and oxidant mixture 206 to proceed to the perforated flame holder102. In another embodiment, a fuel control valve, blower, and/or dampermay be used to select a fuel and oxidant mixture flow rate that issufficiently low for a start-up flame to be jet-stabilized; and uponreaching a perforated flame holder 102 operating temperature, the flowrate may be increased to “blow out” the start-up flame. In anotherembodiment, the heater 228 may include an electrical power supplyoperatively coupled to the controller 230 and configured to apply anelectrical charge or voltage to the fuel and oxidant mixture 206. Anelectrically conductive start-up flame holder may be selectively coupledto a voltage ground or other voltage selected to attract the electricalcharge in the fuel and oxidant mixture 206. The attraction of theelectrical charge was found by the inventors to cause a start-up flameto be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 228 may include an electricalresistance heater configured to output heat to the perforated flameholder 102 and/or to the fuel and oxidant mixture 206. The electricalresistance heater can be configured to heat up the perforated flameholder 102 to an operating temperature. The heater 228 can furtherinclude a power supply and a switch operable, under control of thecontroller 230, to selectively couple the power supply to the electricalresistance heater.

An electrical resistance heater 228 can be formed in various ways. Forexample, the electrical resistance heater 228 can be formed fromKANTHAL® wire (available from Sandvik Materials Technology division ofSandvik AB of Hallstahammar, Sweden) threaded through at least a portionof the perforations 210 defined by the perforated flame holder body 208.Alternatively, the heater 228 can include an inductive heater, ahigh-energy beam heater (e.g. microwave or laser), a frictional heater,electro-resistive ceramic coatings, or other types of heatingtechnologies.

Other forms of start-up apparatuses are contemplated. For example, theheater 228 can include an electrical discharge igniter or hot surfaceigniter configured to output a pulsed ignition to the oxidant and fuel.Additionally or alternatively, a start-up apparatus can include a pilotflame apparatus disposed to ignite the fuel and oxidant mixture 206 thatwould otherwise enter the perforated flame holder 102. The electricaldischarge igniter, hot surface igniter, and/or pilot flame apparatus canbe operatively coupled to the controller 230, which can cause theelectrical discharge igniter or pilot flame apparatus to maintaincombustion of the fuel and oxidant mixture 206 in or upstream from theperforated flame holder 102 before the perforated flame holder 102 isheated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operativelycoupled to the control circuit 230. The sensor 234 can include a heatsensor configured to detect infrared radiation or a temperature of theperforated flame holder 102. The control circuit 230 can be configuredto control the heating apparatus 228 responsive to input from the sensor234. Optionally, a fuel control valve 236 can be operatively coupled tothe controller 230 and configured to control a flow of fuel to the fueland oxidant source 202. Additionally or alternatively, an oxidant bloweror damper 238 can be operatively coupled to the controller 230 andconfigured to control flow of the oxidant (or combustion air).

The sensor 234 can further include a combustion sensor operativelycoupled to the control circuit 230, the combustion sensor beingconfigured to detect a temperature, video image, and/or spectralcharacteristic of a combustion reaction held by the perforated flameholder 102. The fuel control valve 236 can be configured to control aflow of fuel from a fuel source to the fuel and oxidant source 202. Thecontroller 230 can be configured to control the fuel control valve 236responsive to input from the combustion sensor 234. The controller 230can be configured to control the fuel control valve 236 and/or oxidantblower or damper to control a preheat flame type of heater 228 to heatthe perforated flame holder 102 to an operating temperature. Thecontroller 230 can similarly control the fuel control valve 236 and/orthe oxidant blower or damper to change the fuel and oxidant mixture 206flow responsive to a heat demand change received as data via the datainterface 232.

FIG. 5 is a diagrammatic side-sectional view of a portion of a flarestack 500, according to an embodiment, that includes a perforated flameholder 102 substantially as described with reference to FIGS. 2-3. Theflare stack 500 can include a housing 502 in which a flare burner 504 ispositioned. The housing can enclose the combustion volume 204, andincludes an inlet 506, an outlet 508, and vent louvers 510.

The flare burner 504 can include the perforated flame holder 102 and aplurality of fuel nozzles 218 configured to produce fuel streams 206directed toward respective portions of the input face 212 of the flameholder. A fuel line 512 can extend into the housing 502 via the inlet506, and is coupled to the plurality of fuel nozzles 218 and configuredto deliver fuel to the nozzles.

During operation, fuel, such as, for example, waste natural gas from anoil well, may be introduced via the fuel line 512 to the plurality offuel nozzles 218, which emit respective fuel streams 206 toward theperforated flame holder 102. A combustion reaction 302 can be supportedby the fuel streams 206 and held substantially within perforations 210of the flame holder 102. Products of the combustion, such as, forexample, heated air, carbon dioxide (CO₂), water vapor (H₂O), etc., exitthe housing 502 via the outlet 508, whence they are dispersed in theatmosphere. Because the combustion reaction 302 is substantiallycontained within the perforations 210 of the flame holder 102, no flamesare visible outside the housing.

As shown in FIG. 5, the flare stack 500 can be positioned at the top ofa pole or stack, which serves to distribute the combustion products intothe atmosphere at a height that allows them to dissipate

Two fuel nozzles 218 are shown in the embodiment of FIG. 5. However,this is provided merely as an example. According to an embodiment, aflare stack 500 is provided, employing a single fuel nozzle. Accordingto another embodiment, a flare stack is provided that includes a largernumber of fuel nozzles. For example, FIG. 7 shows a retrofit flare stackthat includes an array of fuel nozzles, as described below in detail.According to another embodiment, the flare stack 500 of FIG. 5 includesa similar array of fuel nozzles.

FIG. 6 is a diagrammatic side-sectional view of a portion of a flarestack 600, according to an embodiment, that is similar in many respectsto the embodiment of FIG. 5. The flare stack 600 includes a flare burner602 that includes a fuel nozzle 218 with a variable aperture 606. Thefuel nozzle 218 can include a control element 604 and a nozzle outlet610. The control element 604 can be coupled to an actuator element 608that can be configured to move the control element vertically, therebyregulating the degree to which the control element occludes the nozzleoutlet 610.

The size of the fuel nozzle aperture 606 may correspond to the area ofthe opening, as viewed in transverse section, through which the fuelstream 206 exits the fuel nozzle 218. In embodiments that includeconventional fuel nozzles, the size of fuel nozzle aperture 606 istypically substantially equal to the area of the corresponding opening.However, in the embodiment shown in FIG. 6, the size of fuel nozzleaperture 606 is equal to the area of the nozzle outlet 610 minus thearea of the control element 604 bisected by a plane defined by thesmallest diameter of the fuel nozzle outlet 610. As the actuator 608moves the control element 604 upward, a larger area of the controlelement 604 may be bisected, reducing the size of the fuel nozzleaperture 606. Conversely, as the control element is moved downward, thesize of the fuel nozzle aperture 606 may increase.

In applications where the fuel supply to a flare stack may vary overtime, a fixed fuel nozzle aperture may be problematic. A reduction inthe fuel supply can result in a corresponding reduction in fuel streamvelocity. As discussed with reference to FIG. 2, according to anembodiment, the velocity of the fuel stream 206 is preferably such thatit cannot independently support a stable flame between the fuel nozzleand the flame holder. Under certain conditions, if the velocity of thefuel stream is too low, a flame can begin to burn in the fuel streambefore it reaches the flame holder, which would interfere with properoperation of the flame holder, and would tend to increase undesirableemissions, such as NO_(x). In such situations, it would be desirable toincrease the velocity of the fuel stream 206, in order to cause theflame to be held in the perforated flame holder 102.

It is well understood that the velocity of a fluid passing through anopening is a function of the volume of fluid passing per unit of time,and the size of the opening through which it passes. Velocity rises indirect relation to fluid volume, and in inverse relation to the openingsize. Thus, with reference to the embodiment of FIG. 6, if duringoperation, the fuel supply drops, tending to reduce velocity of the fuelstream, a corresponding reduction in the size of the fuel nozzleaperture 606 will produce an increase in fuel stream velocity, andvice-versa.

During operation, a fuel stream 206 may exit the fuel nozzle 218 andsupport a combustion reaction 302 within the perforated flame holder102, substantially as previously described. If an increase in velocityof the fuel stream is required, such as when a drop in the fuel supplyto the fuel nozzle 218 causes a reduction in velocity, the actuator 608can be controlled to reduce the size of the fuel nozzle aperture 606,thereby increasing velocity. Likewise, where desired or required, theactuator 608 can be controlled to increase the size of the fuel nozzleaperture 606 to reduce fuel stream velocity.

According to an embodiment, the actuator 608 is controlled by a pressureregulator feedback mechanism, in which changes in the fuel supplyproduce corresponding changes in fuel pressure. The regulator feedbackmechanism is configured to respond to these changes by increasing thesize of the aperture 218 as fuel pressure increases, and by reducing thesize of the aperture as fuel pressure decreases.

According to another embodiment, a controller is provided, configured tocontrol the size of the fuel nozzle aperture 606 in response to changesin one or more of fuel pressure, fuel stream velocity, flametemperature, flame position, emission composition, etc.

According to a further embodiment, the actuator 608 is configured to becontrolled by an operator during operation of the flare stack 600.

As previously noted, the perforated flame holder 102 is typicallypreheated prior to normal operation. According to another embodiment,the actuator 608 is controlled to reduce fuel stream velocity during astart-up procedure to permit a flame to be supported by the fuel stream206 between the fuel nozzle 218 and the flame holder 102, in order toheat the flame holder. Once a portion of the flame holder reaches aselected temperature, the actuator may be controlled to reduce the fuelnozzle aperture 606 and increase fuel stream velocity, causing the flameto rise to the flame holder 102.

In FIG. 6, the fuel nozzle 218 is shown as having a separate housing 612that is configured to be coupled to a stack or pipe, and to which thehousing 502 is in turn coupled. According to other embodiments, the fuelnozzle 218 is enclosed within the housing 502 or within the stack, justupstream of the housing.

In FIG. 6, the perforated flame holder 102 is shown occupyingsubstantially all of the cross sectional area of the flare stack.According to other embodiments, as shown, for example, in FIG. 5, theperforated flame holder 102 occupies less than the entire crosssectional area of the flare stack. In some cases, it may be beneficialto configure a flare stack system such that no circulation of gasesaround the perforated flame holder is permitted, while in other cases,such circulation may be advantageous. Accordingly, the determination ofthe size and shape of the perforated flame holder, in relation to thehousing, is a design consideration.

According to an embodiment, the perforated flame holder occupies between⅔ and 100% of the cross sectional area of the flare stack. According toanother embodiment, the perforated flame holder occupies approximately ⅔of the cross sectional area of the flare stack. According to a furtherembodiment, the perforated flame holder occupies between ⅓ and ⅔ of thecross sectional area of the flare stack. According to an embodiment, theperforated flame holder occupies the minimum cross sectional area of theflare stack necessary to maintain sufficient combustion of the volatilecompound.

FIG. 7 is a diagrammatic side-sectional view of a portion of a flarestack 700, according to an embodiment, that includes a retrofit burner702 installed in a pre-existing flare stack. In the example shown, thepre-existing flare stack includes a fin-tube burner 704, which in turnincludes a plurality of fuel tubes 706 extending substantially parallelto each other—along axes that lie perpendicular to the plane of thedrawing—through transverse-oriented fin plates 708, one of which isshown. Each of the plurality of fuel tubes 706 can have a respectiveplurality of fuel nozzles 710 interleaved with the fin plates 708. Inoperation, as fuel is ejected from the fuel nozzles 710, it can entrainair passing between the fin plates 708, and a gas flare is supportedinside the housing 502 and close to the fin-tube burner 704.

According to an embodiment, the retrofit burner 702 includes a pluralityof fuel nozzles 218 coupled to a common fuel line 512. The fuel nozzles218 are interleaved between fuel tubes 706 of a start-up fin-tube burner704. Each of the plurality of fuel nozzles 218 can be configured toprovide a fuel stream 206 to a respective portion of the perforatedflame holder 102. Four fuel nozzles 218 are shown in the view of FIG. 7,but the plurality of fuel nozzles can include an array of fuel nozzlesextending beyond the plane represented in the drawing.

According to an embodiment, during a start-up procedure of the flarestack 700, the fin-tube burner 704 is operated in a mode in which fuelis ejected from the fuel nozzles 710 and a flame is supported below theperforated flame holder 102, which serves to pre-heat the flame holder102. The fuel supply to the fuel tubes 706 is then cut off, and a fuelsupply is supplied to the fuel line 512. Fuel streams 206 are emittedfrom each of the plurality of fuel nozzles 218, and a combustionreaction 302 is ignited and held in the perforated flame holder 102.

As discussed above with reference to FIG. 6, in some applications, thefuel supply can vary. Thus, according to an embodiment, valves areprovided, and configured to individually control flows of fuel to eachof the plurality of fuel nozzles 218. As the fuel supply increases ordecreases, a corresponding number of the plurality of fuel nozzles 218may be brought online or shut down, as necessary. According to anembodiment, the fuel supply to each of the nozzles is controlled sothat, when additional fuel nozzles are to be brought online, only fuelnozzles that are immediately adjacent to currently operating nozzles areactivated. Heat from combustion supported by the adjacent fuel nozzleswill enable a newly activated fuel nozzle to come up to normal operationvery quickly, avoiding extended warm-up time during which unburned fuelmight pass through the flame holder.

Embodiments are described and shown in a stack configuration, i.e., aconfiguration in which the respective systems are supported somedistance above the ground. However, other embodiments are envisioned, inwhich similar structures are positioned on the ground.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments are contemplated. The various aspects andembodiments disclosed herein are for purposes of illustration and arenot intended to be limiting, with the true scope and spirit beingindicated by the following claims.

What is claimed is:
 1. A device, comprising: a housing including aninlet configured to be coupled to a waste fuel supply as part of a flarestack, and an outlet configured to release products of combustion to theatmosphere; a perforated flame holder positioned inside the housing andhaving a first face, a second face lying opposite the first face, and aplurality of perforations extending through the perforated flame holderbetween the first and second faces; and a nozzle configured to receive aflow of fuel from the inlet, and to emit a fuel stream toward the firstface of the perforated flame holder.
 2. The device of claim 1, whereinthe housing is configured to be freestanding, supported only by acoupling at the inlet.
 3. The device of claim 1, wherein the housing isconfigured to be coupled to the flare stack and to be supported thereby.4. The device of claim 1, wherein the nozzle is coupled to receive theflow of fuel via a fuel line extending through the inlet into thehousing.
 5. The device of claim 1, wherein the nozzle is one of aplurality of nozzles, each configured to receive a flow of fuel from theinlet, and to emit a fuel stream toward a respective portion of thefirst face of the perforated flame holder.
 6. The device of claim 5,wherein each of the plurality of nozzles is coupled to receive therespective flow of fuel via a common fuel line extending through theinlet into the housing.
 7. The device of claim 6, further comprising aplurality of fuel valves operatively coupled between the common fuelline and a respective one of the plurality of nozzles and configured toindependently control operation of the respective nozzle.
 8. The deviceof claim 1, further comprising: a start-up burner positioned within thehousing; and a retrofit burner positioned within the housing, theretrofit burner including the perforated flame holder and the nozzle. 9.The device of claim 8, wherein the start-up burner includes a pluralityof burner nozzles arranged in an array within the housing.
 10. Thedevice of claim 9, wherein the nozzle is one of a plurality of nozzlesof the retrofit burner, interleaved among the array of burner nozzles ofthe start-up burner.
 11. The device of claim 10, wherein the start-upburner is a fin-tube burner, and wherein the plurality of nozzles of theretrofit burner is interleaved among fin plates of the fin-tube burner.12. The device of claim 8, wherein the start-up burner is configured tosupport a flame between the start-up burner and the perforated flameholder of the retrofit burner.
 13. The device of claim 1, wherein thenozzle includes an aperture having a size that is variable.
 14. Thedevice of claim 13, wherein the nozzle is configured to regulate avelocity of the fuel stream.
 15. The device of claim 13, furthercomprising an actuator operatively coupled to the nozzle and configuredto control the size of the aperture.
 16. The device of claim 13, whereinthe nozzle includes a nozzle outlet and a control element, the controlelement being positioned to occlude some portion of the nozzle outlet,and wherein movement of the control element varies a degree to which thenozzle outlet is occluded by the control element.
 17. A method,comprising: outputting a waste gas and supplemental fuel sufficient toraise a heating value of the waste gas plus supplemental fuel to about100 BTU per cubic foot or less toward a perforated flame holder; andcombusting the waste gas and supplemental fuel substantially within aplurality of perforations extending through the perforated flame holder.18. The method of claim 17, wherein the combusting waste gas comprisesemitting a fuel stream that includes the waste gas from a nozzlepositioned within a flare stack and toward the perforated flame holder.19. The method of claim 18, further comprising, prior to performing thecombusting waste gas substantially within a plurality of perforationsextending through the perforated flame holder, preheating the perforatedflame holder by operating a start-up burner positioned within the flarestack.
 20. The method of claim 19, comprising, after performing thepreheating the flame holder: shutting off a flow of fuel to the start-upburner; and introducing a flow of fuel to the nozzle.
 21. The method ofclaim 18, wherein the emitting a fuel stream from the nozzle positionedwithin the flare stack is comprised by emitting a fuel stream from eachof a plurality of nozzles positioned within the flare stack, each towarda respective portion of the perforated flame holder.
 22. The method ofclaim 21, wherein the emitting a fuel stream from each of the pluralityof nozzles positioned within the flare stack comprises selecting anumber of the plurality of nozzles based upon a volume of waste gas tobe combusted.
 23. The method of claim 22, wherein the selecting a numberof the plurality of nozzles based upon the volume of waste gas to becombusted comprises varying the number of the plurality of nozzles inresponse to changes in the volume of waste gas.
 24. The method of claim18, comprising varying an aperture size of the nozzle in response tochanges in a volume of waste gas to be combusted.
 25. The method ofclaim 24, wherein the varying an aperture size of the nozzle comprisesvarying a degree of occlusion of an outlet of the nozzle.
 26. The methodof claim 17, wherein the perforated flame holder is positioned within aflare stack; and further comprising venting products of the combustionto the atmosphere.